The US embassy in Japan has advised its citizens to avoid bars in Tokyo's Roppongi entertainment district, popular with tourists, warning of a sharp rise in drink-spiking incidents.

The US embassy in Japan has advised its citizens to avoid bars in Tokyo's Roppongi entertainment district, popular with tourists, warning of a sharp rise in drink-spiking incidents.

"The number of reports of US citizens being drugged in bars has increased significantly in recent weeks," the embassy warned online and in a rare email alert after similar warnings by the British and Australian embassies.

"Typically, the victim unknowingly drinks a beverage that has been secretly mixed with a drug that renders the victim unconscious for several hours," the embassy said.

The alert said commonly "large sums of money are charged to the victim's credit card or the card is stolen outright" before the drugged person wakes up in the bar or on the street.

The US embassy has recommended that its staff avoid Roppongi bars and clubs and said American citizens "may choose" to follow suit or otherwise "remain extra vigilant" and "maintain a high level of situational awareness".